As many as 1000 different odor receptors are coded in the human genome, and although only approximately 350 types are functional, they represent the largest population of G protein–coupled receptors in the genome. The olfactory mucosa also contains somatosensory receptors of the trigeminal nerve. When performing clinical tests of olfaction, clinicians must avoid activating these somatosensory receptors with thermal or noxious stimuli, such as the ammonia used in “smelling salts.”
The olfactory chemoreceptor cells are located in the olfactory mucosa, a specialized part of the nasopharynx. Olfactory chemoreceptors are bipolar nerve cells (
Fig. 8.30 ). The cThe conscious perception of flavor, particularly of foods, is the result of both olfactory and gustatory input based on directly inhaled odor, taste from the food as it is macerated in the mouth, and retronasal odor from the volatile molecules that are released by maceration and pass up into the nasal cavity from the pharynx.
CHAPTER 8 The Special Senses 157 nonmotile cilia on the apical surface of these cells contain chemoreceptors that detect odorant chemicals dissolved in the overlying mucus layer. From its opposite side, the cell gives off an unmyelinated axon that joins other olfactory nerve filaments and penetrates the base of the skull through openings in the cribriform plate of the ethmoid bone. These olfactory nerves synapse in the olfactory bulb, a portion of the cerebral hemisphere of the brain located at the base of the cranial cavity, just below the frontal lobe (
Fig. 8.31
Humans have about 10 million olfactory chemoreceptors. Like taste cells, olfactory chemoreceptors have a short life span (about 60 days), and they are also continuously replaced. However, olfactory receptor cells are true neurons and, as such, are the only neurons that are continuously regenerated throughout life.
The olfactory mucosa is exposed to odorant molecules by ventilatory air currents or from the oral cavity during feeding. Sniffing increases the influx of odorants. The odorants are temporarily bound in mucus to an olfactory binding protein that is secreted by a gland in the nasal cavity.
Olfactory coding resembles taste coding in that most natural odors are complex and consist of many molecules that excite a wide variety of olfactory chemoreceptors. Coding for a particular perceived odor depends on the responses of many olfactory chemoreceptors, and the strength of the odorant is represented by the overall amount of afferent neural activity.
The initial synapse of the olfactory pathway is located in the olfactory bulb, which is a specialized portion of the cerebral cortex and located on the underside of the frontal lobe. It contains mitral cells, interneurons (granule cells; periglomerular cells), and distinct synaptic clusters (glomeruli; see
Fig. 8.31 ) in which theses interact with olfactory afferent fibers. As the olfactory afferent fibers reach the olfactory bulb from the olfactory mucosa, they branch as they approach an olfactory glomerulus to synapse on the dendrites of mitral cells. Each glomerulus is the target of thousands of olfactory afferent fibers, but all the afferent fibers to a single glomerulus convey input from the one type of olfactory receptor. This is all the more remarkable because olfactory receptor cells are being regenerated continuously and new axons must therefore navigate their way to a correct glomerulus.